u used a rebirth water to " reborn " your pet , his savys will be increased , so all the stats (strength etc etc etc ) will be improve , so for example

pet without rebirth at lvl 50 has ( for example ) 50 of strength with 1 rebirth at lvl 50 it will have 57 or wathever ( jsut an example )

The rebirth makes u pet goes to lvl 1 tho

The first rebirth is at lvl 50 ( but its recommended to have ur pet at lvl 80 + ) cause if the first rebirth is at lvl 50 and ur pet is lvl 90 all the extra exp starting from lvl 50 will stil be there and so u can lvl it up to wathever lvl u can reach with the exp from 50 to 90 ( for example )

Also pet can only be rebirth 8 times,
First rebirth require pet to be lvl 50 then lv 80 lv100 and lvl 120, (notice thats only 4 times, so the other four times must be lvl 120 as well.) and thats 8 rebirths

Once rebirth your pet will go back to lvl 1, but if you rebirth your pet at lvl 50, 80, or 100 all of the exp will go back to the pet.

example: u rebirth a lv 70 pet, the first rebirth is lv50. So lets pretend it took 40mil exp to lvl ur pet from lvl 50 to 70, that means after you rebirth all that extra 40mil exp will go back to your pet.

Also notice that your pet doesn't have 8 rebirth... that means you need an item called "rebirth water" to add an extra rebirth to your pet.